Ruiz played in two rehab games at Lehigh Valley over the weekend, which convinced him that he was ready.
"I felt really good at home plate, and that surprised me because I hadn't faced a pitcher for a long time," Ruiz said. "I had some good at-bats and feel like I am ready to go."
This has been a difficult season for Ruiz, who began it with a 25-game suspension for testing positive for Adderall, a stimulant. Ruiz entered Tuesday hitting .235 with two RBIs in 51 at-bats.
"He stayed in Florida before the season, and when he joined us he had trouble getting going," manager Charlie Manuel said. "I think the fact is that major-league pitching is better than the pitchers he was seeing."
Manuel said he wanted to ease Ruiz back into action, which is why he was batting so low in the order.
Chase Utley said Monday that he hoped to begin a rehab assignment within 48 hours. If the weather had cooperated, he would have gone Tuesday.
Manuel said the rain kept Utley from going to Reading.
"No sense in going if it is raining," Manuel said.
The second baseman has been on the disabled list since May 23 with a right oblique strain.
Reading is home through Sunday.
Top pick signs
The Phillies have signed first-round pick J.P. Crawford, a shortstop from Lakewood High in Southern California. Crawford was the 16th player selected in the first round. The Major League Baseball slot value for the 16th pick is $2.3 million.
Ryan Howard belted his eighth home run of the season Monday, snapping a string of 71 plate appearances without going deep.
The first baseman has been playing on a sore knee, and Howard said it was giving him trouble again Monday.
Manuel believes that if Howard can stay in the lineup on a regular basis, his power numbers will increase significantly.
"If he gets 500 at-bats or 550, you will see: I think he is very capable of hitting 35 home runs, especially if he hits .280 or .290," Manuel said.
Howard entered Tuesday's game with a .274 average.